Structural adhesives replace welds and mechanical fasteners in many applications because structural adhesives increase fatigue life and reduce failure commonly found around welds and fasteners. Structural adhesives can also be preferred over welds and mechanical fasteners where resistance to flex and vibration is desired.
When structural adhesives are applied to substrate surfaces, a bond line forms at the meeting of the substrate surfaces. It is critical for optimal performance in these cases for the bond line to have uniform thickness.
When a substantial force is applied, structural adhesives used in adhesive bonding may be loaded and fracture (1) normal to the bond line, which can create a peeling effect causing substrate materials to be on different planes (i.e., peel fracture), or (2) perpendicular to the leading edge of a fracture, whether in-plane or out-of-plane, which creates a shearing effect where substrate materials remain on the same plane (i.e., shear fracture). While fracturing is typically avoided, if there is to be fracturing, shear fracture is preferred over peel fracture because shear fracture requires more external loading than peel fracture to produce failure.
Solder material, in the form of solder elements, can be added to adhesives to improve bonding with substrate surfaces. However, impurities (e.g., dirt) and films (e.g., oxide) formed on substrate surfaces and solder material can prevent the solder elements from adequately bonding to the substrate surfaces, preventing sufficient interlock.